a) Field of the Invention
The present invention relates to manufacture of semiconductor devices, particularly to those devices having fine patterns.
b) Description of the Related Art
For the manufacture of semiconductor devices and the like, photolithography techniques are used for transfer a fine pattern on a photomask to a photoresist film. Light diffraction and interference cannot be neglected if a pattern size is near the same order as the wavelength of exposure light. For example, if a distance between adjacent openings becomes short, light fluxes passed through the openings are diffracted and interfered each other. Therefore, the exposure level at a photoresist film is raised at some points in the area corresponding to the area between the adjacent openings and in neighboring areas, so that the adjacent openings cannot be resolved into separate openings.
As one method of solving this problem, a Levenson type phase shift mask has been proposed. This mask uses a phase shifter which gives one of adjacent openings a phase difference of .pi. radian to make light fluxes passed through the two adjacent openings have opposite phases. Diffracted and interfered composite light of opposite phases weakens its intensity so that the adjacent openings can be resolved into separate openings.
Levenson type phase shift masks have, however, fatal restrictions. For example, if there are three openings A, B and C and the openings B and C are assigned an inverse phase relative to the opening A, the openings B and C are necessarily assigned the same phase. It is therefore theoretically impossible to make all adjacent openings to be assigned an inverse phase relative to each other.
Design changes of such contradictory areas are therefore necessary. LSI's, especially, have generally a very large number of circuit elements disposed in as small an area as possible. Even one design change affects other circuit areas, and if design changes of a plurality of circuit areas are to be made, an enormous number of design change choices are possible. It is also difficult to predict whether one design change produces better or worse results of the whole circuitry.
If a designer manually makes such design changes, a long time work is required in addition to experiences and knowledge. In order to use a modest tool for semi-automatic design change provided by CAD makers, data required for using it should be prepared beforehand and a number of work processes are also required.
It has been long desired to automatically change designs of contradictory areas of a physical layout of a semiconductor device.